A group of genetic disorders with widely different clinical manifestations predispose affected individuals to a high risk of cancer. Four of these disorders are rare, recessively transmitted traits, they are: ataxia telangiectasia (A-T), Fanconi anemia (FA), Bloom syndrome and xeroderma pigmentosum (XP). The first three disorders are collectively called chromosome-breakage syndromes, and they share the characteristic of chromosome instability, while the fourth disorder, XP, is characterized by high sensitivity to sun exposure and susceptibility to skin cancer. Skin fibroblasts or blood lymphocytes from individuals with these disorders show increased sensitivity to mutagens and DNA-damaging agents, suggestive of defects in DNA repair; such a defect has, in fact, been established in XP.
In addition to these rare disorders, certain neoplasms are also hereditary with distinct patterns of Mendelian behavior in man. Five of these disorders, namely, familial polyposis, Gardner syndrome, retinoblastoma, Wilms'tumor and hereditary cutaneous malignant melanoma (HCMM) (with its precursor lesion, the dysplastic nevus) are inherited as autosomal dominant traits. These tumors occur less commonly in the hereditary than the nonhereditary form, tend to develop earlier in life and to arise from multiple foci. In addition to these dominantly inherited neoplasms, there are numerous other cancers which tend to occur in clusters with a familial tendency (see e.g., Fraumeni, J.D. Jr. "Clinical Patterns of Familial Cancer". Chaganti et al. (eds.), Genetics in Clinical Oncology, pp. 223-233 (New York: Oxford University Press (1977)).
In examining skin fibroblasts from individuals with hereditary tumors or familial cancers, a common abnormality in their response to irradiation has been observed during the G.sub.2 phase of the cell cycle. Exposure to X-rays (100R) or cool-white fluorescent light (8 W/M.sup.2) produces chromatid breaks and gaps observable in the first post-treatment metaphase. The frequencies of these aberrations have been found by the present inventors to be several-fold higher in skin fibroblasts from individuals in whose families the previous disorders are present (genetically predisposed individuals), than in comparable cells taken from clinically normal (control) individuals. Similar observations have also been reported with respect to skin fibroblasts and peripheral blood lymphocytes from individuals with A-T or FA (see e.g., Rary et al. "Cytogenetic Studies of Ataxia Telangiectasia". Am. J. Human Genet., 26:70A (1974); Taylor, A.M.R. "Unrepaired DNA Strand Breaks in Irradiated Ataxia Telangiectasia Lymphocytes Suggested from Cytogenetic Observations". Mutat. Res., 50:407-418 (1978); Bigelow et al. "G.sub.2 Chromosomal Radiosensitivity in Fanconi's Anemia". Mutat. Res., 63:189-199 (1979); and Natarajan et al. "Chromosomal Radiosensitivity of Ataxia Telangiectasia Cells at Different Cell Cycle Stages". Hum. Genet., 52:127-132 (1979)). Additionally, cultures of human blood lymphocytes taken from cancer patients, including many with hereditary cancers, and treated during G.sub.2 phase with the radiomimetic chemical bleomycin generally showed a higher frequency of chemically-induced chromatid damage than comparable cells from healthy, normal donors (Hsu et al. "Differential Mutagen Susceptibility in Cultured Lymphocytes of Normal Individuals and Cancer Patients". Cancer Genet. Cytogenet. 17:307-313 (1985)).
The inventors have observed this abnormal response to G.sub.2 irradiation in all the human tumor cells they examined, irrespective of histopathology or tissue origin. The same response has also been seen in human keratinocytes in culture prior to neoplastic transformation. In somatic cell hybrids produced by fusion of a normal skin fibroblast with a tumor cell, HeLa, the enhanced chromosomal radiosensitivity was suppressed and segregated with the tumorigenic phenotype. Cytogenetic and biochemical studies with and without DNA repair inhibitors suggest that the enhanced chromatid damage seen during the post-irradiation period results from deficient DNA repair processes during G.sub.2.
From the observations and results discussed above, it was theorized by the inventors that based on the magnitude of the chromatid damage which resulted from exposure of cells in culture to X-rays or visible light, and the association of such damage with genetic disorders predisposing an individual to cancer or hereditary neoplasms, a quantitative assay for determining genetic predisposition or susceptibility to cancer could be developed. The inventors believed that an accurate and reproducible assay could be formulated based on the observation that the phenotype associated with cancer predisposition appears to result from deficient DNA repair processes during G.sub.2 phase. This repair deficiency(ies) would lead to genetic instability which, in turn, would increase the probability of mutations such as inactivation or loss of the normal allele at a heterozygous cancer-predisposing locus. Thus, the inventors endeavored to develop a quantitative assay for identifying cancer susceptibility, and, further in that regard, to determine what assay parameters would affect the cellular responses to X-irradiation in such a way as to produce false positives.